Method and apparatus for continuously measuring a dimension of a moving object



c. BURNS ETAL 2,670,651 ARA R CONTINUOUSLY MEASURING A DIMENSION OF AMOVING OBJECT 3 Sheets-Sheet 1 March 2, 1954 METHOD AND APP TUS F0 FlledAug 14 1950 AMPLIFIER HIGH SPEED 2 mwmm Hm h m 4 E 2 M Ms DE 2 EW.\ R H0/JJ 5 SM... ,0 H o H R 2 mm mm \UML in M, 7 3 7 Q L m m /w/ l 4 1 1 7 66 .M M 4 4 1 1 2 Kl s 2 M 6 I 6 4 M 4 m 4w l II Il'i' L n s a e bINVENTORS: M M W ATTORNEYS- S ETAL March 2, 1954 v Q B N 2,670,651

METHOD AND APPARATUS FOR CONTINUOUSLY MEASURING A DIMENSION OF A MOVINGOBJECT Filed Aug. 14, 1950 3 ShQets-Shet 2 OOOOOOOOOOO 0 0 O O O O O O OO O INVENTORS: M M

W ATTORNEY5- March 2, 1954 c BURNS ETA], 2,670,651

METHOD AND APPARATUS FOR CONTINUOUSLY MEASURING A DIMENSION OF A MOVINGOBJECT 3 Sheets-Sheet 5 Filed Aug. 14, '1950 ATTORNEYS- Patented Mar. 2,1954 METHOD APPARATUS FOR CDNTINU" OUSLY MEASURING A DIMENSION OF MOVINGoBJEc'r Charl sBurnsand S m el S- .Corli le, Londo England, assignors toThe British Iron & Steel Research Association, London, England, aBritish c mpany Application August 14, 1950, Serial No. 179,30 1

Claims priority, application 1 Great Britain August 18, .1949

2 Claims. (81 88 -14) This invention relates tomethods and appara-- tusfor continuously measuring a dimension of a moving object andmoreparticularly for continuously measuring the width of a moving stripor plate. One object of the invention istonobtai'n accurate measurementsof a dimension of .a moving'object irrespective of any bodily movement.of the object in the plane of or normal to vthe direction of thedimension.

For a better understanding of the invention one basic method ofmeasuring a dimension of a moving object in accordance therewith willfirst be described briefly, before consideration is given to specificarrangements of apparatus for performing the methods of measurement. Forconvenience the method will be described with reference to themeasurement of the width of a moving strip or plate, though theinvention is not limited to such objects or dimensions.

In this basic method a light beam is formed by collimating the lightfrom a source so 'asto form va parallel beam. This beam is directed pastone edge of the object in a direction perpendicular to its width. Thebeam is then optically directed so that it eventually passes theopposite edge of the object again in a direction perpendicular to itswidth. The optical'path or" the ibeamis arranged so that an odd numberof lateral inversions take place between the two edges of the object.The side of the beam furthestfrom the first edge r the object will thenbecome 'theside nearest to the second edge of the object, as it passesby the latter. If the axis of the beam is placed near enough .to eachedge of the object,

by suitable arrangement of the optical apparatus,

then one edge of the object will intercept a portion of the beam on oneside, and the second edge of the object intercept a portion of the beamon its other side. The width of the beam after it has been curtailed byboth edges (hereafter called the residualbeam) f will be relatedinversely to the width of the object, if the beam has a uniform iwidthof cross section along the direction of the edges of the object. As. thewidth ofbeam 'intercepted by the object is the sum or the effects *ofthe two edges, a movement ofthe object parallel to its width will notalter the width of. the residual beam. Likewisemovement perpendicular tothe Width will have no effect since thebeam is parallehjtheselconditions being bothslibiect to. certain limitations on. the angle rdivergence f of the b amcause' by he finite s ze f he real or virtuallight source. Thus any variation in the-width of. the object. willappear as variation "is the total amount or light carried by theb'eamthe two slits S1 and 52' are covered and-uncovered 2 which can readilybe detected, photo-electric arrangement.

in practice it is generally desirable to also use a comparison beamwhich comes from thesame source as is used'to form the measuringbeainalready described. The comparison beam'is automatically modulated sothat it conveys to the photo-"electric receiver, or othermeasuringmeans, as much light flux as' is carried'by themeasuring beam. Thedegree of vmodulationis then proportional to the width or the body.Electrical and mechanical arrangementslmay' bejprovide'd which permit ofthe use of a singlephotocell to measure the light fiux carried by "bothbeams for example. by a individually and also automatically controls thecomparison beamto maintainlequality between the beams. At thesame timethe means of controlling the comparison beam is made to indicate, e. g.by an electric c-urrent or the mechanical movement of a shutter thewidth of the cohlparh son beam, which'being kept in a constant ratio tothe width of the measuring beam, serves as a .measure of strip width.

showing the construction of this first arrange- .ment.

Figure 3 is a circuit diagram of the electronic portion of the apparatuswhich is the'sain'e in vboth arrangements.

Figure 4 s: schematic diagram of the second arrangement. 40

Referring first to Figure 1 light from a lamp I0, is collim'ated byalens L1 to'fall on a slit S1 and by lenses L2, after reflection by amirror'M1, to fall on a slit S2. In'front of the slits {S1 and there ismounted av rotatable chopper {disc I I having the form' of a toothedwheel, the angular width of each tooth, and; each gap between teeth,being equal to the angular distance 'betweenthe slits, which are soorientated as to "beparallel to the tooth edges. Accordingly,whenth'e'-'cl'1opper disc [I is rotatedby a motor (not shown),

alternately, the light falling on one when it "is cutoff from the other;The two beams from the virtual sources iormed by the slits Stand $2,"the latter after being turned th'rou'gh'a right angle acvacai by mirrorM2, are made parallel by lenses La and L4, to form the measuring beam 52and the comparison beam 13 respectively. The measuring beam :2 isdirected past one edge [4 of the strip I5 whose width is to be measuredand is curtailed thereby. The partly curtailed measuring beam I2 isthereafter reflected through a right angle by the prism P1 and aftertravelling in a direction parallel to the width of the strip is againreflected through a right angle by a pentagonal prism P2 and directedpast the second edge it of the strip l5 laterally inverted with respectto the strip 15. That is, the other side of the beam 42 from that whichwas curtailed by the edge 5:2 is thus curtailed by the edge It. This isdue to the combination of the right angle prism Pi and the pentagonalprism P2 which provide anodd number of lateral inversions. It will beappreciated that variations in the width of the strip i5 will beaccompanied by variations in the residual superposed, since thephotocell cathode is placed in the focal plane of lens L5, and the beamsenter the lens L5 parallel to one another. A difiusing plate I8 isinterposed in front of the photocell ll,

to spread the illumination over a greater area of the cathode.

The arrangement of lens L5 and the beams and photocell ll ensures that apencil of light made up of any part of the cross section of either beam,irradiates the photocell cathode over substantially the same area, withsubstantially the same intensity distribution. This ensures thatmovement of either beam transverse to its direction, occasioned byshaking of the strip 15 in the measuring beam I 2 and movement in thecomparison beam of a shutter l9 (whose function will be describedbelow), causes no difference in photocell response through an overallshift of the light intensity distribution pattern over the surface ofthe cathode.

Before reaching lens L5 the comparison beam i3 is varied in width alongone dimension of its cross-section by the shutter 19, already referredto, which is spiral in shape and is mounted on the spindle 2%}- of ahigh speed meter movement (H) of the moving coil centre zero type. Thisis supplied by current, in a manner to be explained hereafter, in such away as to maintain equality between the light flux falling on thephotocell from the residual portions of the comparison beam 43 and themeasuring beam I2.

Owing to the action of the chopper disc H the photocell I7 isalternately illuminated, for equal intervals of time, by light from theresidual portions of the comparison and measuring beams. If these areunequal, an alternating voltage will be set up across a load resistor(not shown in Figure 1) placed in the cathode of the cell. The phase ofthis alternating voltage will differ by 180 according as the light fluxin the measuring comparison beam is the greater.

The alternating voltage from the photocell ii is amplified by an A. C.amplifier 22 of normal design whose output is fed to a phase sensitiverectifier stage 23. The D. C. output is amplified in a. D. C. outputstage 2! and. the amplified output of one sign or the other is fed tothe highspeed meter movement 2|, which is of the centre zero type. Themeter movement it thereupon moves the spiral shutter H to increase ordecrease the cross-section of the comparison beam 13, in the directionwhich tends to reduce to zero the difierence between the light fluxcarried by the residual portions of the two beams i2 and i3. By makingthe amplifier of high gain, the beams can be made equal to a very highaccuracy, and the angular position of the spiral shutter It can becalibrated in terms of strip width. Moreover a continuous width recordcan be provided by means of a pointer and pen 25 co-operating with achart recorder 26 in known manner.

The construction of the arrangement just describedwill now be describedwith reference to Figure 2 of the drawings. In this particularconstruction the instrument is designed to measure the width of hotsteel strip whose mean width may lie between 4 and 16 inches, thevariation on any given width being up to i /g inch. The accuracy ofmeasurement required is $0.02 inch, but greater accuracies can beachieved by the instrument described. The strip may normally pass atabout 20 iii/second, and may move bodily /3 inch in the direction of itswidth, and i /a inch at right angles to it.

The instrument is built in two halves, to be located at each side of thestrip. The two halves are each built on a heavy base-plate 39, 31respectively on which all the components are mounted and are covered bysteel boxes (not shown) which fit over flanges 32 and 33. The lamp it}fits into a socket 34. The lenses L1, L3, and slit S1 are contained in ameasuring beam collimator housing 35, and the lenses L2, L4, slit S2 andmirror M2 in a comparison beam collimator housing 36. The mirror M1 ismounted on a support 31 which is rigidly connected to the housing 38.

Also on the baseplate 30, but on the side remote as shown in thedrawing, is an electric motor 38 driving the chopper disc 38, whichprojects through a slot 49 cut in the baseplate 33. The chopper disc hassix teeth, and rotates at a speed adjustable by a small resistance (notshown) in series with the motor 38, to chop the light beams at afrequency between 200 and 300 cycles per second. The chopper disc may be7 in diameter, with teeth cut deep.

The lamp It! may conveniently be a Pointolite small source lamp,operated by direct current.

On the same side of the plate as the chopper motor 38 is mounted areference frequency signal generator (represented by the rectangle 4|and shown diagrammatically in Figure 3 to be described hereafter) forsupplying electronically an electrical pulse of the same frequency asthe chopping of the light beams. This is required by the phase sensitiverectifier stage described above. The image of a small flash lamp bulbfilament is formed on a portion of the teeth of the chopper disc I ilying within the broken lines 41, behind which is placed a photoelectriccell, the output of which is applied to a one-stage valve amplifier. Theoutput of this amplifier is an alternating voltage of the same frequencyas the chopping frequency of the light beam, and by moving the positionof the image of the bulb filament on the chopper disc the phase of thisvoltage can be adjusted.

The beams pass from one box to another through the telescopic tubes 42,43, which permit of the baseplates 30, 3| being spaced apart at asuitable distance to accommodate the strip width chosen. a

0n the second baseplate 3| the measuring beam c-gorog'csi ls'reflectedkby ithe xprism Pa, and :the mirrorithrou'gh *the'ilensLswhich also receives=tho com- :parison beam. ihe lens Ls may 'bearectangular plastic lens, sufficientlyilargeto-admit both beams :sidebysideat maximum-width. The comparison beamis controlled by: thes'spiralshutter l 9 which consists of .a spiral of :thin i'foil amounted on anpiderof thin: brassLflangedifor -.rigidity, and fixed toa bushfl-awhichffits over-'the spindlezrzil of the imeterzmovement; 2 I

The: photocell 1' l 1 iis mounted: in a shielding box fflaand itsoutputis amplifiedbytheni'c.amplifier -22. Themoxes housingkthe phasesensitive firec- :Jtifier; stage1li23 and zthezlli C. outputystage'flton :thereverse side of:baseplate 3-|,are also shown.

tfrhe ,applioation'of:thisinstrument' to the meas- .urement ofzthe widthrofxsteel strip 1 involves the 'measuring beamv 152 ipassing ,throughtheopen air beside each'stripredge. 'There isponecessitwfor thecomparison-beam: l 3 to go? outside the-.instru ment. @The measuring. abeam I'2 passes through windows #6 :of :heat ;resisting ;;glass, overwhich pass ietsofuair-iromdetsfl, otherjets -flrserving to remove: theenvelope of. heatedsairaround each edgo which ;otherwise.-gives rise ;to--refraction. of thevmeasurin beam.

.tai-1s:of th electronic circuits ar -shown in Fi ure 3 and -:as-theseare typical circuits for performing their appropriate function andform-no part of thejinventioniperise, it is unnecessary to idiscus'sttlaemimdetail. :lB-riefiy, valve V1 is a first amplifier stagefed from the load resistor of the photocell l'ltand feedinga:pushspull-amplifying and phaseesplitting stageieenstitutedby the twovalves V2 and V3. These in turn feed a phase sensitive rectifier stageVgVs'i the suppressor grids of-which are'fed"withreference frequencysignals from -the generator comprising valve Va. This iattervalveis'fedfrom'the' output of aphotocell tswhichreceive light-interruptedtbythe chopper disc '1 i in the "manner previously described. ValvesV6$V1-constitute theD.-"C. output stage, their anodes beingbrought-tojopposite ends of the coil ,5!) of the meter movement 2!, theqfintreof fihfi-coil'iill. going to H. T. positive. As already explainedwhen an alternating voltage of the chopping frequency is produced therectifier stage raises the D. C. voltage on the grid of one of thevalves V6, V7 and lowers it on the other thus producing an unbalance inthe anode currents passing through the coil 50 which is thus deflectedone Way or the other.

Although the jets 41 and 48 (Figure 2) may in most circumstances preventdimming of the windows 46, in some cases dimming may occur resulting ina permanent and spurious unbalance in the beams giving rise to an errorin width reading whilst dimming obtains.

The second arrangement to be described employs a modified optical systemfor use where dimming may occur and which is designed to avoid or reducethe effect thereof.

The electronic circuits employed and the general arrangement of theapparatus are similar to the first arrangement so that the secondarrangement will only be described with reference to the schematicdiagram of Figure 4 in which similar parts to those in Figure 1 aresimilarly referenced.

The optical system is modified so that the measuring and comparisonbeams l2 and I3 are transmitted through the same external windows, andsubstantially the same areas of these, so that any general dimming ofthese glasses which reduces the transmission, though not being soserious as to render the instrument inoperable, will tend to *balanceouti'in 'thetwo beams-andreducecr eliminate anyiaccompanying error.

Asibefore, the images of the lampifilament :are thrown :on slits S1, 182.by :lenses Ll and 1L2. in this ,1 case however :the light passingthrough/ S1 forms J-the comparison: beam 1| 3 and that "passing through:Sathe-measuringbeam :12. The lens L4, is placed at twice itsfocal.:length.:fromt:S1;so thatran image of S1 .is :formed at an equal.1 dis- "tanlceson-- therothertside of lens "This'fecuszlP isplacedintheapproximate plane ofthe strip 1 I25, and slightlyoutsideithelimit: of. movementthere.- of. It isztermedP: in Figureal.

Considering the :beam'from Sathis is reflected by a rightiangled mirrorand. passes through a. lens "L3; which :forms 'an image of slit 55g "on.srnall mirror "ii/[0,. surface coated. Mirror 'Mn is fixed one narrow.-mount spanning -thei'beam which is spassingbetween. slit S1 andlensi'Lc.1-. so :that it will not affectthe.linearity of beamziil-lumeination i against width .Irequlred 1 by the principle of .itheinstrument and the g obstructioneiformed will not affect the action ofthe comparison beam. Mirror Mo being placed in the focal ip-lape of161153114 the: light .brought: to sfocus-at it ,will be :ma'deparaliel:bydens Th4 :and form rthe measurimg:1-heam.i2asrbefore, one sideheingi-ofi by the first-edge l 410i thestripkfi.

'Afterpassingi by. the stripedge k6,:both

i ii. and: I3 are receivedzbyca lens: La-andpassput below the secondstrip edge from :a 'similar lens iLmartoiflbeing turnedthroughmtwoi-right angles by. miIROI'S QMi' anal/I5. These twoIEHSG'SiLs: and IL". are of equal focallength and care'placed .-.a

distance: apart-equal toztwicentheir focal-length.

-l3eca1. .se of ;this,-.2paral'le1 light entering them emerges parallel,having beer-rlproughtto av foous th'e .ifirst in the t focal plane :ofthe second. :Alsm flight :brought to :a focus ..at.a. distance Sin frontofsvlens Ls equalto theiocal lengthcwill :be' broughtto a focus, againequal. distance from: the: lens: Lil. Lens-1L6 is placedat-a distanceequal to: its focal 'llength fromthe pointiP. where the comparison. beaml3i is focussed; Accord.- ingly, the comparison :beam 1 3 passesthrough-1L6 and Lc'an'd is brought toaseoond! tocus at::Q, in acorresponding position close to the second strip edge It. The measuringbeam I2 enters Ls parallel, and so emerges from L7 parallel, having alsobeen given the lateral inversion necessary which was supplied by thearrangement of prisms in the instrument previously described. After Qthe two beams enter lens La, which brings the measuring beam l2 to afocus at the small mirror M; where it is reflected through a right angleon to the lens L9 which makes it parallel once more. The comparison beamI3 is also turned through a right angle by the mirror M7 and is madeparallel by lens Ls. Both beams 12 and I3 are so collimated and madeparallel to each other and enter lens L5 to be focussed on the photocellH, the system after this point being exactly the same as with theinstrument previously described.

This system requires that the optical length of path between points Pand Q should be constant. The adaptability of the system to a variety ofstrip widths, which in the first instrument described is obtained byaltering the separation of the baseplates, is secured by extendingmirrors M4 and M5 and mounting them permanently a fixed distance apart.Lenses L6 and I11 are mounted in a fixed position relative to theremainder of the optical system exclusive of mirrors M4, M5. It will beseen that by this arrangedesired variety of strip widths.

ment the separation of the two chassis can be has no effect onmeasurements as it does not cause variations in photocell output. If thechopper disc is located later in the system stray light, which may beradiation from the hot strip, is modulated by the disc and thereforecauses variations in the photocell output. It is also possible to have asimpler optical system and thebeams can be kept fairly narrow at thechopping position thus giving a sharp rectangular waveform.

We claim:

1. Apparatus for measuring continuously the distance between twoopposite edges of an opaque moving strip of substantial width comprisinga source of light, light projection means positioned on one side of theplane of said strip for forming light from said source into aparallel-sided beam of light having a width less than the distancebetween said edges and for projecting said beam through the plane ofsaid strip at a position overlapping a first one of said edges, so thatone marginal portion only of said beam is intercepted by said strip,optical reflecting means positioned on the opposite side of the plane ofsaid strip to said light projection means for receiving the part of saidbeam which passes said first edge and for directing it through the planeof said strip at a position overlapping the second of said edges, withthat margin of the beam which passed immediately adjacent said firstedge located outside of said second edge, so that another and differentmarginal portion of the beam is intercepted by said strip, and aphotoelectric light flux measuring device disposed on the same side ofthe plane of said strip as said light projection means and positioned toreceive the residual beam passing said second edge.

' 2. Apparatus for measuring continuously the distance between twoopposite edges of an opaque moving strip of substantial width comprisinga source of light, light projection means positioned on one side of theplane of said strip for forming light from said source into aparallel-sided beam of light having a width less than the distancebetween said edges and for projecting. said beam through the plane ofsaid strip at 'a'position overlapping a first one of said edges, so'thata marginal portion at one side of said beam is intercepted by saidstrip, optical reflecting means positioned on the opposite side of theplane of said strip from said light projection means for receiving thepart of said beam which passes said first edge and for directing itthrough the plane of said strip at a position overlapping'the 'secondofsaid edges, with that margin of the beam which passed immediatelyadjacent said first edge located outside of said second edge, so thatanother and difierent marginal portion at the opposite side of the beamis intercepted by said strip, and a light flux measuring devicepositioned to receive the residual beam passing said second edge. A g.

CHHARLES BURNS.

SAMUEL s. CARLISLE- References Cited in the file of this patent UNITEDSTATES PATENTS

